Influence of long term thermal ageing on mechanical properties for In718 & In625.

For structural applications which might be subjected to medium / high temperatures exposition during in-service, the use of superalloys such as In718 or In625 is a common material choice in Aerospace. Justification to extend the Aircraft service-life and the possibility to encounter higher temperature with future Aircraft architectures might lead to particular metallurgical/mechanical evolution of the thermally exposed material. These are valuable reasons to review Long term thermal stability of In718 and In625. Both alloys, being already widely used since decades, have been extensively studied for Jet Engine applications (typically for temperature above 650°C exposed about 10,000 hours) focusing mainly on Static, Stress rupture or Creep strain testing. One could notice that intermediate temperature ranges (~600 to 650°C) for much longer duration (>100,000 hours) have been less investigated, notably due to experimental constraints and lack of industrial emphasis. Next to this, Fatigue, Crack propagation or Fracture toughness properties, which are valuable for Structural design, have also been much less addressed. Based on this background, a project has been initiated to study this particular field of application by mechanically characterizing thermally aged materials (including some over 3 years) to monitor material performance evolution. Metallurgical characterizations have been also performed to try identifying some of the influencing factors impacting the mechanical properties while fractography investigation has been done to identify changes in the failure modes.

Finally, Hollomon & Jaffe (H&J) parameter has been used to attempt predicting the impact of various thermal aging on mechanical properties. Significant differences on the overall material behavior but also among the various mechanical properties have been observed after thermal aging. A parametric law has been proposed to modelise the evolutions of the studied mechanical properties versus H&J parameters.The accuracy of this law and its limitations will be also discussed.